Page 318 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

P1: GQQ/GLT P2: GRB Final Pages
Encyclopedia of Physical Science and Technology EN014B-670 July 28, 2001 16:50

384 Rubber, Natural

Ridley originally developed the herringbone pattern for bedding or carpet backing, and adhesives. The percent-
the tapping panel, shaving 1–2 mm of bark with each cut age is higher in such producing countries as Malaysia and
or tap. The cut is made at an optimum angle of 25 from Liberia.
◦
the horizontal. At the lower end is a vertical channel ter- Latex is concentrated from an original 30% to about
minating in a metal spout below which is attached a latex 60%. Such processes include centrifugation, evaporation,
collecting cup. or creaming. Centrifuged latex accounts for about 90% of
Tapping is usually performed in the early morning the total. The residual serum contains about 5% rubber,
hours, although rainfall will seriously curtail the flow of which is recovered and known as skim rubber.
latex.Afterflowingforseveralhours,thelatexvesselsplug Evaporated latex is prepared by passage through film
up with coagulum. If the cut is reopened the next day, latex evaporators at elevated temperatures. The final concen-
flow begins again. This indicates the rapid regeneration of trate is usually higher than 60% solids and also contains
latex by the tree. the smallest rubber particles.
At each successive tapping, a shaving of bark about Creamed latex is the smallest volume type. It is pre-
1
16 in. thick is made, along the lower edge. This tapping pared by mixing the latex with a creaming agent, such
system is known as spiral tapping. Other tapping systems as ammonium alginate. Creaming occurs as the Brownian
are the “half spiral alternate daily” and the “full spiral movement of the rubber particles slows down. The pro-
fourth daily.” Another unusual system is the double tap, cess may take several weeks. Creamed latex is primarily
on opposite sides of the tree, with one cut about 4 ft higher used in the manufacture of latex thread.
than the other cut.
Latex from the tree (fresh, whole field latex) is 30–40% 1. Yield Stimulation
total solids by weight. The tapper is paid according to the
“drc” (dry rubber content), as measured by a hydrometer. The selection of planting material was the first step in
The contents of each tapping cup are collected in 5 gallon improving the yield of rubber foam trees. This was fol-
containers, then taken to nearby collecting stations for lowed by the application of certain chemicals, called plant
pickup trucks to deliver to large storage tanks at bulking “hormones,” to the bark near the tapping panel. Most com-
stations. This practice is similar to the way dairy farmers mon were 2,4-dichlorophenoxyacetic acid (2,4-D), 2,4,5-
handle milk. tri-chlorophenoxyacetic acid (2,4,5-D), and even copper
A drop of ammonia is usually placed inside each cup sulfate. Such increases in yield are temporary, and treat-
before tapping, to prevent premature coagulation in the ment must be repeated frequently. Yield increases of 25%
cup. Collected latex is further stabilized by the addition of are common, although some areas respond more or less,
about 0.01% ammonia, 0.05% sodium sulfite, or 0.02% say 20–40%.
formaldehyde. Fresh latex has a normal pH of 7. Such stimulants presumably operate by increasing the
Residue in the latex-collecting cup coagulates natu- surface area of bark drained by the tapping cut. Of course,
rally, and is called cup lump. This is salvaged by the this implies a physiological change that is not known. At
tapper during his next tapping, when he also removes least, prolonged use of such stimulants does not appear to
the coagulated latex skin over the tapping cup. This is be harmful to the tree.
called tree lace. Any latex that drops on the ground is col- Inrecentyears,amajorbreakthroughwasaccomplished
lected every few months. Such rubber is named earth scrap in the field of yield stimulation. This stemmed from a
(Table IV). study of the “plugging” process of latex vessels. It now
About 10% of tree latex is concentrated and shipped appears that the difference between a high-yielding tree
to consumer countries, for conversion into finished la- and a low-yielding tree is the length of time of latex flow
tex products, such as a host of dipped goods, foam for before plugging, that is, coagulation at the cut end of the
latex vessel.
Specific chemicals were found that inhibited the clos-
TABLE IV Distribution of Dry Rubber Types
ing off of the vessels. Most effective was ethylene gas,
Source International type Total (%) among a host of others. Today, commercial compounds
that slowly liberate ethylene are used. Among these is one
Chem. coag. latex Pale crepe/smoked sheets 80.0
called Ethrel, 2-chloroethyl phosphonic acid (Table V).
Cup lump, tree lace Estate brown crepes 15.5
The most common application is to prepare a 10% slurry
Bark scrap Thin brown crepes, ambers 2.0
of Ethrel in palm oil. Application by hand with a paint
Earth scrap Flat bark crepe 2.0
brush on the bark near the tapping panel is the next step.
Factory salvage Flat bark crepe or higher 0.5
Within a very few days enough hydrolysis has occurred
Total 100.0
to permit ethylene absorption within the tree tissues. The